Bump-on-Trace (BOT) structures are used in flip chip packages, where metal bumps are bonded onto metal traces in package substrates directly, rather than bonded onto metal pads in conventional package bonding schemes. The BOT structures facilitate smaller chip areas, and the manufacturing cost of the BOT structures is lower compared to conventional package bonding schemes. The BOT structures achieve substantially the same reliability as the conventional bond structures that are based on metal pads.
When using a BOT structure, the metal bumps are soldered onto the metal traces on the package substrate by a reflow process. However, the metal bumps are typically wider than the metal traces, and hence the solder bonding the metal bumps to the metal traces may shift. Several problems may result from the metal bump shift. For example, solder bumps may crack, or may bridge to neighboring metal traces, especially at the minimum bump to trace location causing device failure. Further, the solder bumps may shift and bridge to neighboring metal traces due to the coefficient of thermal expansion (CTE) mismatch between the package substrate and the chip.